Abstract: The seismogenic northeastern North America-Caribbean oblique-slip plate boundary
includes the 8.5-km deep Puerto Rico trench, 120 km north of the densely populated islands of
Puerto Rico and the Virgin Islands. The northern insular margin of Puerto Rico, adjacent to the
Puerto Rico trench, is characterized by frequent seismicity, rapid Neogene trenchward tilting,
and oversteepened slopes. Multibeam bathymetry, sidescan sonar, and single-channel seismic
reflection data reveal extensive submarine landslide deposits on the margin that can be traced
upslope to two large (up to 55-km wide and 6.1-km deep) amphitheater-shaped headscarps along
the edge of the Puerto Rico –Virgin Islands (PRVI) carbonate platform. The crown of the larger,
westernmost scarp incises the platform at 2600 meters below sea level, ~ 40 km off the north
coast of Puerto Rico. The associated submarine landslide deposits extend up to 80 km
trenchward, covering a total seafloor area of 4313 km2. Seismic reflection data show a
maximum debris deposit thickness of 2850 m. The debris deposit consists of multiple layers each
approximately 200 m thick. This suggests that the slope failure may have occurred as multiple
failure events, rather than a single catastrophic event. Allowing for compaction of the debris
deposit sediments, the volume of 1378 km3 for the amphitheater is comparable to the calculated
compacted landslide debris volume of 1426 km3. These results suggest that the collection of
submarine landslide units is associated with the formation of the giant amphitheater over time.
This also suggests that the debris is not being removed by translation or subduction, unlike other
convergent margins. Although the exact triggering mechanism(s) for the submarine landslides is
not known, tectonic erosion related to the westward migration of the overthickened (20+ km)
southeastern Bahamas Province beneath the PRVI margin is a likely candidate. The present-day
collision zone is interpreted to be offshore northwestern Puerto Rico, an area characterized by mid-slope uplift, anomalously high seismicity and crescentic cracks in the PRVI carbonate
platform that are similar in size and shape to amphitheaters observed further to the east. Evidence
of multiple, Pliestocene and younger submarine landslide deposits adjacent to the uplifted Mona
Block and within the Mona Rift suggest that the devastating 1918 tsunami could have been
generated by a seismically triggered submarine landslide. Moreover, the proximity of crescenticshaped
seafloor cracks to the tectonically active Mona Rift make them likely candidates for sites
of future breakaway scarps and catastrophic submarine landslides. A modern day inundation
would have detrimental effects on Puerto Rico whose population has dramatically increased over
the past century to about 3.89 million.